1. Field of the Invention
The present invention relates to a multilaminate material for separating electrode of opposite polarity in electrochemical cell and battery assemblies. Two pieces of the multilaminate material may be sealed along at least two opposite peripheral edges to form a unitary multilaminate separator assembly having an electrode receiving slot. The multilaminate material of the present invention is especially suitable for use in lead-acid storage batteries. It provides improved active material retention and, thus, improves cell performance and extends cell life, and provides improved electrical insulation between electrodes of opposite polarity.
2. Description of the Prior Art
Lead acid secondary cells comprising two substantially lead electrodes in contact with sulfuric acid electrolyte are well known to the art and are utilized in a variety of applications, including starting, lighting and ignition batteries for internal combustion engines and electrical systems, stationary batteries for powering communications systems, electrical utilities, computer systems, and for providing emergency power, motive power batteries to provide power for propulsion of lift trucks, mining equipment, and the like, and special purpose batteries such as aircraft, marine, military, and small, sealed batteries for consumer use. Each of the electrodes must be provided with the appropriate active material prior to cell operation, since it is the active material which provides reactants for the reversible electrochemical reactions taking place during charge and discharge of the cell. The electrodes provide conductive support structures for the active material and serve as current collectors for the cells.
Conventional negative electrodes for use in lead-acid secondary cells comprise thin lead or lead alloy grids. The open spaces and surfaces of the grid are pasted with a paste-like mixture comprising one or more oxides of lead which are subsequently reduced, forming the electrochemically active material known as sponge lead. Positive electrodes may comprise the same type of thin lead or lead alloy grids pasted with a mixture of lead oxide compounds which are subsequently converted to the active material, lead dioxide. A battery is assembled by alternately aligning positive and negative electrodes and inserting insulating separators between and underneath the electrodes to prevent short circuiting during charging and operation.
Pasted electrodes for use in lead-acid storage batteries are conventionally wrapped with multiple layers of porous materials prior to battery assembly to provide improved retention of the active material within the grid and improved electrical insulating properties. The multiple layers of porous materials are typically applied to the electrodes manually by wrapping each layer around the pasted grid electrode in a specified orientation. Typically, a first layer of porous retainer mat, such as non-woven fiberglass mat, is wrapped around the pasted grid electrode horizontally, and a second layer of porous retainer mat is wrapped around the pasted grid electrode vertically. A plastic boot may then be affixed at the bottom of the plate to electrically insulate the bottom edge of the plate. Some prior art processes utilize a third layer of perforated plastic material manually wrapped around the electrode to hold the, retainer mat in place and to electrically insulate the side edges of the electrode. Separators are then inserted between electrodes during battery assembly. The primary disadvantage of conventional battery wrapping techniques is that they are tremendously time consuming and labor intensive. In addition, they require extensive handling of the pasted grids which are fragile and comprise harmful lead components.
U.S. Pat. No. 4,418,464 teaches an apparatus for wrapping plates with three layers of acid resistant, flexible, porous materials. The apparatus first wraps a layer of Retent-A-Tape around the electrode in a top to bottom orientation with respect to the front and back faces of the plate, and the tape is clamped in place with an elastomeric boot. The apparatus then wraps a precut double layer comprising a first layer of fiberglass mat and a second layer of Koroseal around the electrode in a side to side orientation with respect to the electrode faces. The Koroseal layer is slightly larger than the glass mat layer, overlapping edges of the Koroseal are sealed after application, and the bottom edge of the Koroseal layer is then bonded to the boot. The apparatus
taught by the '464 patent is designed to overcome many of the deficiencies of the prior art methods which are described above by replacing manual wrapping with an automated wrapping process. The automated process of the '464 patent, however, requires complex machinery, and requires application of the separators separately from and subsequent to the wrapping operation.
U.S. Pat. No. 4,448,862 teaches coating conventional microporous separators for lead-acid storage batteries with a porous glass wool mat on the side facing the negative electrode to promote electrolyte exchange and prevent crumbling of the active material. A glass wool mat may be applied as a layer to the separator, or it may be formed as a pocket into which the negative electrode may be inserted. U.S. Pat. No. 4,353,969 teaches a quasibipolar battery wherein the separator assembly comprises a separator substrate bonded to loosely woven glass mats on both faces, the glass mats attached to at least one layer of scrim fabric providing reinforcement of the pasted active material and support for the separator-plate assembly. U.S. Pat. No. 3,625,770 teaches a flexible battery separator in the form of an envelope for high energy density batteries, the separator envelope comprising asbestos impregnated with polyphenylene oxide (PPO) to which a composite zirconia-based flexible inorganic separator film is applied.
U.S. Pat. No. 4,476,203 teaches lead-acid battery cells wherein separation of electrode plates is achieved by successive separator elements, including a microporous flat layer adjacent an undulating and perforated spacing element which is adjacent to a fibrous mat. U.S. Pat. No. 3,953,236 teaches a laminated cell structure comprising a positive plate with active material, a fibrous structure adjacent both faces of the positive plate, and a separator layer comprising a microporous sheet between the fibrous layers and the negative electrodes. U.S. Pat. No. 4,618,549 teaches utilizing a retainer mat in addition to the conventional separator to prevent shedding of active material and to prevent the distribution of leady dust during curing. U.S. Pat. No. 4,252,871 teaches a tubular support sleeve for tubular plate electrodes in the form of a seamless helical tubular lattice structure comprising fusible flat reinforced thermoplastic tapes. The '871 patent also teaches the use of a braided glass thread layer supported by an outer sheet of perforated plastic, such as polyvinyl chloride (PVC). U.S. Pat. No. 4,262,068 teaches a maintenance-free lead-acid battery having a separator comprising a double layer of glass fibers and a porous member. U.S. Pat. No. 4,275,130 teaches a bipolar battery comprising microporous, thermoplastic separators having porous, resilient mats bonded thereto to carry the active material.
None of the prior art methods or apparatus achieves effective retention of the active material on the electrode grids in combination with a simplified, economically feasible assembly operation. Prior art manual battery wrapping techniques are labor intensive, delicate, and environmentally unsafe, while prior art automated techniques require complex, expensive machinery. Prior art multilaminate materials have not demonstrated improved retention of active material resulting in superior performance and enhanced cell operating lifetime.